The rest of "San Tan Valley" is considered a part of the Florence School District and the name of the High School is: Poston Butte. Poston Butte is located on Gantzel Road(also known as Ironwood) just Northwest of Bella Vista Road.
Most Arizona schools are willing to accept open enrollment. I live and work in San Tan Valley, my kids go to both the Coolidge and Florence School Districts and I coach softball for San Tan Foothills High School. Feel free to contact me directly with any further questions about the community.